HOW TO USE THE DRIFT METER

by Pierre Verster (Paddy)

Having the Winddrift Meter popped up in your window, you will first of all see a division stretching out at either side of the neutral zero position. You should realize, that the numbers of this division have nothing to do with flightcourses directly, but do indicate the numbers of degrees to add to or substract from your actual compass course after you measured your winddrift.

Looking through the eyepiece straight down to the earth's surface, you see the different objects like roads, railroads, coastlines, buildings, rivers moving from top to bottom. If there were no winds, the objects would move in strictly vertical sense through the eyepiece of the Winddrift Meter from top to bottom, parallel to the engraved lines in the eyepiece, when set to neutral zero degrees position. You can check this out by setting no wind in the FS Weather Menu. Of course measuring drift assumes a clear groundsight, be it flying in clear skies or in between cloud formations

The revolving knob at the right bottom side of the drift meter-gauge bears the inscription WDA (Wind Drift Angle). To determine the WDA, you should turn the revolving WDA-knob in such a direction that the observed moving objects on the earth’s surface will move strictly parallel to and within the 3 engraved lines in the eyepiece of the drift meter-gauge. Having done so, the engraved centerline will point to the current WDA in degrees.
As per convention, the WDA measured at the left side of the neutral zero position is expressed in negative degrees, asking for a WCA (Wind Correction Angle)of the same value with the opposite plus sign. The WDA measured at the right side of the neutral zero position is expressed in positive degrees, asking for a WCA of the same value with the minus sign.

For an example, consider the picture on the right. It shows a WDA to the left of the neutral zero position of -12 degrees. Assuming the desired magnetic course is 300 degrees and the plane is currently on that heading, this indicates that the present groundtrack is now 300 – 12 = 288 degrees magnetic due to wind coming in from starboard. Doing nothing would result in passing the destination to the left.
To compensate for this wind, you will have to correct your present compass course with the WCA, being the WDA with opposite sign, in this case +12 degrees to make good your initial groundtrack of 300 degrees magnetic. So your new compass course becomes 312 degrees magnetic.
Had this same wind come in from port, the WDA would have shown +12 degrees, at the right side of the neutral zero position, indicating that the aircraft had been drifted off to a new groundtrack of 300 + 12 = 312 degrees magnetic. Again the WCA would have been the WDA with opposite sign being -12 degrees to correct your compass course, the latter becoming 288 degrees magnetic in order to make good your initial groundtrack of 300 degrees magnetic.

The result is that your aircraft’s nose is pointing more or less into the direction from where the wind comes, and in doing so it will eliminate the drifting-off windforce, while keeping the aircraft on the desired track. Of course measuring drift assumes a clear groundsight, be it flying in clear skies or in between cloud formations.

There exist some different methods for using the drift meter, depending on the manufacturer of the gauge. As you can see in the picture, the letters L and R are used, while we use the method with the + and – signs, leading to quite the same results.

In the left bottom corner of the drift meter-gauge you will notice a digitized value stating -12 in the picture. This has been brought in by the designer for your convenience. The figure represents the WDA you measured and by giving it the opposite sign, you will obtain your WCA for correcting your magnetic course with to eliminate the current winddrift.

Determining your Ground Speed (GS) and ETA
Having measured the course of the groundtrack between A an B on a chart with Mercator projection and having corrected this course for mean magnetic variation enroute to steer your course as per compass, you can determine the distance between A an B using the chart’s scale. Look for any groundmark in the chart that crosses your intended groundtrack or that will pass abeam your port or starboard wing between A and B, such as a river, a highway, a village, a town or a railroad. These will be your “pinpoints”. Now suppose you have found on your chart a river and a highway, both crossing your groundtrack and that you have measured them to lay 16 nm apart on your groundtrack.
Note these pinpoints and their distance apart. Note also the distance between your second pinpoint and your destination B. Having reached your assigned altitude, correct your magnetic course as soon as possible for the prevailing winddrift. On approaching your first pinpoint enroute, (the river) set the driftmeter sight and push the START button of your timer at the moment of overflying this pinpoint. Do the same reaching the highway pinpoint, but now push the STOP button of the timer.
Suppose the elapsed time between those two pinpoints shows 06:30 indicating an elapsed time of 6.5 minutes, then you can calculate the current GS as to be 60 / 6.5 x 16 nm = 148 knots. Having noted from the chart your distance between your second pinpoint (the highway) and your destination B and looking at your panelclock, you can figure out your Estimated Time of Arrival (ETA) over B, if winds on flightlevel do not change much.

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